The prior art, notably U.S. Pat. No. 3,845,018 to N. Bilow et al, and U.S. Pat. No. 3,879,349, also to N. Bilow et al, discloses certain acetylene end-capped polyimide oligomers and describes methods for their preparation. The polyimide oligomers are useful as heat-curable coatings on wires or other substances, as laminating resins, and as molding resins. Laminates and moldings prepared from such oligomers have low void contents as well as excellent thermal and physical properties.
The reported processes for preparing such products are inefficient and costly. In the firste step of the process, an aromatic tetracarboxylic and dianhydride is reacted with an aromatic diamine to form an anhydride capped polyamic acid. This intermediate product then is reacted with an aminoarylacetylene to product an acetylene-capped polyamic acid. A solvent which will form an azeotrope with water, e.g., benzene, or react with water, e.g., acetic anhydride, is added to the acetylene capped polyamic acid product and the midture is heated to remove water from the amic acid groups to form imide groups. This reaction is run under reflux conditions with water being removed to drive the reaction to completion. The aromatic tetracarboxylic anhydride and the polyimide formed in the reaction both have low solubilities in common solvents and for this reason it is necessary to employ special high boiling solvents such as dimethyl formamide (DMF) or N-methyl-2-pyrrolidone (NMP) in the process.
In addition to their high cost, the prior art processes produce products having recognized deficiencies. Specifically, the high boiling solvents employed in the processes are difficult to remove completely. Unless removed virtually completely, residual solvent evolves when the acetylene end-capped polyimide oligomers are cured. This action causes moldings prepared from such oligomers to fracture.
For the above reasons, there is a need in the art for an improved process for the manufacture of acetylene end-capped polyimide oligomers.